I. Field of the Invention
The present invention relates to valves which respond to the presence of a particular substance in fluid flowing therethrough, and more particularly to hydrocarbon monitoring valves for use with oil storage tanks.
II. History of the Prior Art
In oil tanks having floating roofs, such roofs are typically provided with a drainage system to prevent a buildup of rain water and other fluids thereon. The drainage system is usually disposed beneath the floating roof and within the oil in the tank, and often consists of a flexible line coupling a drain at the center of the roof to the tank wall adjacent the bottom thereof. The flexible line permits the floating roof to rise or drop within the tank as the quantity of oil stored therein varies.
An example of a flexible drain system for use with a floating roof in an oil tank is provided by U.S. Pat. No. 4,790,446, which patent issued Dec. 13, 1988 to Roger W. Thiltgen. The drain system described in the Thiltgen patent employs rigid pipe lengths joined together by flexible joints. Each flexible joint is comprised of a pair of side frames having pairs of leg portions pivotally coupled to each other along an axis and mounting a length of flexible pipe therebetween. The opposite ends of the length of flexible pipe are coupled to an opposite pair of pipe couplings mounted on central portions of the side frames between the leg portions and adapted to be coupled to the lengths of rigid pipe. The flexible joints are designed to undergo pivoting movement through a substantial angular range without restricting or pinching off the length of flexible pipe utilized therein. This allows the floating roof to move between a maximum height of fifty feet or more and a minimum height of just a few feet above the tank bottom while maintaining the drain system open.
The drain system exits the lower tank wall via a length of pipe which is typically positioned to drain into a containment dike at the base of the tank. The containment dike which serves to contain oil in the event of an inadvertent spill, also receives rain water discharged from the roof drain system. A gate valve, located in the roof drain system just outside of the lower tank wall, controls the discharge of rain water from the drain system. The gate valve may be left open, in which event rain water is continually drained by the roof drain system. Alternatively, the gate valve may be kept closed except when it rains, following which the valve is opened long enough to discharge the rain water which has accumulated on the floating roof.
Because of environmental requirements and restrictions, it is important that none of the hydrocarbon product contained within the tank be discharged to the outside of the tank, where it could evaporate into the atmosphere or otherwise pose a contamination problem. For this reason, floating roof drain systems are designed and constructed with considerable care. Nevertheless, it is always possible for oil contained within the tank to leak into the drain system. Flexible collapsing drain systems often have numerous joints and other portions thereof which are vulnerable to leakage in the presence of the surrounding oil.
It would therefore be desirable to provide, for use with the roof drain system, a valve capable of sensing the presence of hydrocarbons in the rain water or other fluid product being discharged by the roof drain system and to close in the event that hydrocarbons are detected.
It is known to provide valves which close upon detection of the presence of a particular substance. Examples of such valves are provided by U.S. Pat. No. 3,491,911 of Fraylick et al., U.S. Pat. No. 4,294,276 of Harrison, U.S. Pat. No. 3,586,018 of Bogardh, et al., U.S. Pat. No. 3,472,253 of Herscher et al., and U.S. Pat. No. 2,630,346 of Karlssen. The Fraylick et al. patent provides an example of a hydrocarbon sensing valve for use with the roof drain system of an oil tank. The valve described in Fraylick et al. employs a foam block in conjunction with a spring-loaded shaft having a valve element mounted at on end thereof. The valve element normally remains in the open position. However, when hydrocarbons ar present in the fluid being discharged through the valve, the foam block deteriorates and this allows the spring to move the shaft and close the valve element.
Hydrocarbon sensing valves such as the valve described in the Fraylick et al. patent represent one solution to the problem of preventing discharge of oil from an oil storage tank by the floating roof drain system. However, there is considerable room for improvement in the design of such valves. For example, presently known valves tend to require the presence of a significant amount of hydrocarbon material before the hydrocarbon-sensitive element begins to deteriorate.
In the valve of the Fraylick et al. patent, a dam is provided so that the discharging fluid can build up in the region of the foam element and immerse the foam element in the escaping hydrocarbon product. Without the dam, the volume of fluid draining through the valve could be small enough to allow the hydrocarbon product to escape through the roof drain system undetected. However, the presence of the dam presents problems of its own as does the presence of any obstruction in valves of this type. Because of the presence of the dam, the valve can easily clog or otherwise malfunction.
Still other problems characterize valve designs such as that shown in the Fraylick et al. patent. For example, it is frequently desired to provide an improved and more positively operating arrangement for closing the valve element in response to the presence of hydrocarbons. Still other areas where improvement can be made relate to the shape and design of the hydrocarbon-sensitive element.